1. Field of the Invention
The present invention relates to a resistor element and, particularly, to a high precision resistor element to be used in an impedance matching.
2. Description of the Prior Art
With a recent speed up of a system control, various systems for high-speed interface have been proposed and standardized in semiconductor devices. In many of these interface systems, resistor elements are used for terminating transmission lines.
It has been usual that such resistor element is mounted on a substrate together with a semiconductor device. However, with the recent popularization of high speed interface, it has been requested to incorporate such resistor element in the semiconductor device since the resistor element tends to increase the mounting area. FIG. 5 is an illustrative plan view of a conventional resistor element incorporated in a semiconductor device. In FIG. 5, a first terminal 48 formed by a first wiring layer 47 is connected to one end of a resistor layer (WSi layer) 41 formed in a semiconductor device through a first contact 46 and a second terminal 49 formed by a second wiring layer 45 is connected to the other end thereof through a second contact 42.
However, it is required that a resistance value of a resistor element for impedance matching or termination is highly precise. FIG. 6 is a circuit diagram including a resistor element connected in series between a buffer and a wiring of a mounting substrate. Considering, for example, a case where a resistor element 51 formed in a semiconductor device such as shown in FIG. 6 is series-connected between an output buffer 52 in the semiconductor device and a wiring 53 on a mounting substrate, the purpose of the resistor element 51 is to correct an internal impedance of the output buffer 52 to thereby match it with a characteristics impedance of the wiring 53. It becomes possible to restrict noise caused by reflection due to increase of signal speed, by precisely performing this impedance matching. Therefore, the resistance value of the resistor element must be highly precise.
When such resistor element is incorporated within the semiconductor device, it is difficult in a conventional layout shown in FIG. 5 to guarantee the resistance value. That is, since the resistance value R of the resistor element having layout shown in FIG. 5 is determined by a formula R=xcfx81sxc3x97L/W where L is length of the WSi layer, W is width of the WSi layer and xcfx81s is sheet resistance, a change of outer configuration (L and W) due to variation of process such as etching process influences the resistance value directly.
An object of the present invention is to provide a highly precise resistor element, which is hardly influenced by variation of size thereof due to process.
A resistor element according to the present invention comprises a resistive layer provided on a semiconductor substrate through a first insulating film, a first wiring layer provided on the resistive layer through a second insulating film, a second wiring layer provided on the first wiring layer through a third insulating film, a group of first contact regions provided in the second and third insulating films for electrically connecting the resistive layer to the second wiring layer and a group of second contact regions provided in the second insulating film for electrically connecting the resistive layer to the first wiring layer. The second contact regions are provided on and along a circular line or a polygonal line having a center registered with a center point of the first contact region group.